Sigma receptors (σ) have received much attention from the drug discovery field due to their possible involvement in schizophrenia, regulation of motor behavior, convulsions, anxiety, and the psychostimulant effects of drugs of abuse including cocaine, methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA).1,2 In addition to a host of neurological and psychiatric areas of interest, sigma receptors are promising drug development targets for, oncological, immunological, cardiovascular, opthalmological, developmental, gastrointestinal and metabolic disorders as well as those affecting the endocrine system. They are structurally unique proteins that are distinct from classical G protein-coupled receptors, ionotropic receptors, or receptor tyrosine kinases. With two subtypes currently known, they modulate cell survival and excitability, and subserve many critical functions in the body. Endogenous ligands for these receptors are unknown, though current clues point to neurosteroids.3 
The two subtypes, σ-1 and σ-2, were delineated by studies examining their respective molecular weights, distribution in tissue and drug selectivity profiles. The 223 amino acid σ-1 protein with two transmembrane spanning regions has been purified and cloned from several animal species including mouse, rat, guinea pig, and human.4-8 To date, the σ-1 receptor is well studied and known because of the receptor sequence information and availability of selective σ-1 ligands. But, the protein corresponding to σ-2 sites has not yet been cloned. Also, σ-2 receptor-selective ligands are less common, with tritiated DTG (1,3-di(2-tolyl)guanidine) being accepted as a radioligand in the presence of (+)-pentazocine (to block binding to σ-1 sites). Due to the lack of availability of detailed protein structural information and truly selective σ-2 ligands, the pharmacological characterization of the σ-2 subtype has been very limited. There is clearly a need for a selective σ-2 ligand which can not only act as a probe to explore unknown biochemical mechanisms, but also be used as a radioligand in σ-2 receptor binding assays.
The abuse of drugs is a serious social, economic and health problem worldwide. Some of the opiates, cocaine, amphetamines and phencyclidine (PCP) are the drugs of abuse with significant affinities for σ receptors. Current treatments for drugs of abuse are limited and there is a need to develop novel and effective agents to combat this problem.
Cocaine use and abuse has been reported as early as the late 1500 s.9 The historical use has been associated with the chewing of leaves from the Erythroxylon coca bush, from which cocaine was isolated in 1860,10 to eliminate fatigue in workers. Indeed, cocaine is a powerful and addictive psychostimulant. Cocaine abuse is widespread and is responsible for more serious intoxications and deaths than any other illicit drug. However, the invigorating effects of cocaine have caused it to become a major recreational drug of abuse throughout the world with an estimated 13 million people using the drug. In 2004, 34.2 million Americans aged 12 and over reported lifetime use of cocaine with approximately 5.6 million reporting annual use and an estimated 2 million reporting current use of the drug. In 2004 alone, there were an estimated 1 million new users of cocaine amounting to ˜2,700 per day. Despite a decline between 2002 and 2003 which is thought to potentially be due to increases in usage of other stimulants such as methamphetamine, data from the National Survey on Drug Use and Health showed near a 70% increase in the number of people receiving treatment for cocaine addiction from 276,000 in 2003 to 466,000 in 2004.11 
Currently, there are no approved medications to treat cocaine abuse or addiction. An effective strategy used to develop an anti-cocaine agent was the development of antagonists that compete with cocaine for its target proteins. For years, treatment approaches have targeted the dopaminergic system which is known to be involved in the actions and rewards of cocaine use. Many compounds were generated and tested that targeted the dopamine transporter which was identified as a primary site of action of cocaine. These compounds were met with very limited success as many of them just substituted for cocaine.12 After many years of investigation at the dopamine transporter as well as the dopamine receptors, researchers have been challenged to envision novel mechanisms that may afford new therapeutic interventions for cocaine addiction.
Although many other mechanisms are under investigation, the a receptor system has been demonstrated and validated as a legitimate target for the attenuation of cocaine effects. The ability of cocaine to bind to the sigma receptors was discovered and first documented in 1988.13 It was reported that cocaine had micromolar affinity to the sigma receptor, and this interaction corresponded to micromolar levels that were achievable by cocaine in the body.14 Additional studies have indicated that reducing brain sigma receptor levels with antisense oligonucleotides attenuates the convulsive and locomotor stimulant actions of cocaine. Synthetic small molecule antagonists for sigma receptors have also been shown to mitigate the actions of cocaine in animal models. From prior work, the role of the σ-1 subtype has been clearly linked to the actions of cocaine. However, the role of the σ-2 receptor has been suggested, but is less clear due to the lack of truly selective ligands for this subtype.
Radioligands selective for σ-1 receptors have the potential to non-invasively detect and monitor various pathologies, including neurodegenerative diseases and cancer. Applicants herein report the synthesis, radiofluorination and evaluation of a new 18F fluorinated σ-1 receptor ligands including 6-(3-fluoropropyl)-3-(2-(azapan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one (18, [18F]FTC-146). [18F]FTC-146 displays superior in vitro affinity and selectivity compared to other reported σ-1 receptor compounds. The new 18F fluorinated σ-1 receptor ligands, including [18F]FTC-146, can be synthesized by nucleophilic fluorination using an automated module. [18F]FTC-146 afforded a product with >99% radiochemical purity (RCP) and specific activity (SA) of 3.9±1.9 Ci/μmol (n=13). Cell uptake studies revealed that [18F]FTC-146 accumulation correlated with levels of σ-1 receptor protein. Furthermore, the binding profile of [18F]FTC-146 was comparable to that of known high affinity σ-1 receptor ligand (+)-[3H] pentazocine in the same cell uptake assay. PET images of [18F]FTC-146 in normal mice showed high uptake of the radioligand in the brain which is known to contain high levels of σ-1 receptors. Time activity curves (TACs) showed rapid, high initial uptake of [18F]FTC-146 in the mouse brain. Pre-treatment with non-radioactive CM304 (1 mg/kg) reduced the binding of [18F]FTC-146 in the brain at 60 min by 83% denoting that [18F]FTC-146 accumulation in mouse brain represents specific binding to σ-1 receptors. These results indicate that [18F]FTC-146 is a good candidate radiotracer for studying σ-1 receptors in living subjects.
Initially the sigma receptor was thought to belong to the opioid class of receptors;15 however, further studies classified it as a distinct molecular entity, resulting in its recognition as a separate family of receptors.16 There are at least two a receptor subtypes, the σ-1 and σ-2 receptors.17 The σ-1 receptor is the best characterized of the two at present.18, 19 
Despite initial controversy and conflicting ideas, recent key discoveries concerning the a receptor have helped elucidate various biological aspects about this molecular chaperone and its putative functional roles.20,21 Mainly located at the endoplasmic reticulum of cells, σ-1 receptors have been implicated in a host of biochemical processes and pathological conditions including neurodegenerative diseases, psychiatric disorders, drug addiction, digestive function, regulation of smooth muscle contraction and ischemia.20, 22-24 σ-1 receptors are also highly expressed in most known human cancers (e.g., breast, lung, colon, ovarian, prostate, brain).24,25 Agonists for σ-1 receptors influence intracellular and extracellular Ca2+ levels and thus have a broad range of neuromodulatory effects.26,27 Certain σ-1 receptor agonists have been shown to regulate endothelial cell proliferation,28 improve cognition,29,30 provide neuroprotection,31 and act as anti-depressant agents,18,32 while antagonists inhibit/attenuate cocaine-induced seizures,33 highlighting the potential of σ-1 receptors as both a diagnostic and therapeutic target.
There are a multitude of compounds that target σ receptors, including three specific classes of compounds; 1) benzomorphans, such as (+)-pentazocine (FIG. 1) and (+)-N-allylnormetazocine (NANM) that preferentially bind σ-1 receptors (compared to their (−)-enantiomers), 2) endogenous neurosteroids like progesterone (an antagonist of the σ-1 receptor) and 3) butyrophenones, such as the antipsychotic agent haloperidol that displays high affinity for both a receptor subtypes.19,34 Over the last two decades numerous groups have reported the development of high affinity σ-1 receptor ligands34-42—and of these, some have been labeled with radioisotopes (FIG. 1) for use in positron emission tomography (PET) studies.
Examining σ-1 receptors in living subjects with PET is an important step towards understanding the receptor's functional role and involvement in disease. PET radioligands specific for σ-1 receptors could potentially provide a non-invasive means of 1) visualizing and investigating the machinery of these sites, 2) assessing receptor occupancy (to help determine optimal doses of therapeutic drugs), 3) early detection and staging of σ-1 receptor-related disease(s), and 4) monitoring therapeutic response. Some existing σ-1 receptor radioligands include: [11C]SA4503,43 [18F]FM-SA4503,44 [18F]FPS,45 [18F]SFE,46,47 [18F]FBFPA,48 [18F]fluspidine49 and [11C]1339 (FIG. 1). The high affinity σ-1 receptor radioligand [11C]SA4503 has demonstrated promising results in rodents,43 felines50 and non-human primates,51 and is currently the only σ-1 receptor radioligand being routinely used in clinical research;52, 53 however, it is far from ideal for several reasons including its high non-specific binding, affinity for other sites such as emopamil binding protein (EBP),54 and suboptimal kinetic profile (indicative of irreversible binding). The fluorinated derivative of [11C]SA4503 (known as [18F]FM-SA4503) has demonstrated similar disadvantages in rodents and non-human primates, and is yet to be evaluated in humans. The piperidine [18F]FPS reported by Waterhouse and colleagues was evaluated in human subjects in 2003,46, 55, 56 however it displayed unfavorable kinetics (due to its inability to reach transient equilibrium at 4 h p.i.). Following these results, a lower affinity fluoromethyl derivative of [18F]FPS (known as [18F]SFE) was developed in hope of rectifying the issue of irreversible binding.46 Whilst [18F]SFE exhibited a superior kinetic profile (cleared from rat brain with a 40% reduction in peak uptake over a 90 min period), it was found to have a lower selectivity ratio, and in fact blocking studies in rats using a selective σ-2 receptor compound resulted in a small yet noticeable reduction in [18F]SFE uptake.46 In 2005 Mach and colleagues reported the radiosynthesis of another piperidine derivative [18F]FBFPA (affinity for σ-2 receptor/σ-1 receptor=44) and demonstrated its ability to bind σ-1 receptors in both rodent and rhesus monkey brain.48 In 2010 the synthesis of a spirocyclic piperidine σ-1 receptor radioligand, [18F]fluspidine, and its evaluation in mice was reported.37, 49 Biodistribution results showed 40% reduction in brain [18F]fluspidine uptake over 2 hours, indicating that it may display reversible binding; however, it is still in the early stages of evaluation. Moussa and colleagues published the radiosynthesis of a carbon-11 labeled N-benzyl piperazine σ-1 receptor ligand, [11C]13, and its in vivo evaluation in Papio hamadryas baboons using PET imaging. Whilst [11C]13 accumulated in sigma-1 rich regions of the brain and peripheral organs, it was found to display a low selectivity ratio (affinity for σ-2 receptor/σ-1 receptor=38) and also nanomolar affinity for 5-HT2B receptors.39 
At present, there is still no highly selective σ-1 receptor radioligand labeled with fluorine-18 or carbon-11 available for clinical research.